CATHODOLUMINESCENCE SCANNING ELECTRON-MICROSCOPY OBSERVATIONS OF (SRBACA)TIO3 CERAMIC VARISTORS

(Sr0.35Ba0.35Ca0.30)(1.026)TiO3 ceramic varistors were produced with a nd without an additional reducing heat-treatment (PO2 = 10(-12) Pa, 11 00 degrees C, 2 h) in the sintering process (PO2 = 10(-8) Pa, 1400 deg rees C, 2 h) followed by reoxidizing process (900 degrees C, 4 h) in a ir. The CL spectra of the varistors exhibit a broad luminescence, havi ng a maximum at 465 nm, which correlates to the transition between sin gle oxygen vacancy let el and valence band. CL images reveal that the grain, particularly near the varistor surface, has a high-conductive g rain zone rich in oxygen vacancies and a less-conductive grain boundar y zone poor in them. The boundary zones may work as the double Shottky barriers in the varistor. The additional heat-treatment produces thic k boundary zones and consequently gives rise to high varistor coeffici ent (>4), favourable for practical use. It has also been found that a glassy phase (SiO2BaO-MgO) is segregated at boundary enclosed by three grains.